The following table provides the amortized cost and fair value by major categories of securities available for sale carried at fair value. The net unrealized gains (losses) are reported on an after-tax basis as a component of cumulative OCI. There were no securities classified as held to maturity as of the periods presented.

• Includes collateralized debt obligations with a cost basis and fair value of $543 million and $674 million, respectively, at March 31, 2013, and $556 million and $644 million, respectively, at December 31, 2012.
• Included in the “Other” category are asset-backed securities collateralized by auto leases or loans and cash reserves with a cost basis and fair value of $5.6 billion and $5.7 billion, respectively, at March 31, 2013, and $5.9 billion each at December 31, 2012. Also included in the "Other" category are asset-backed securities collateralized by home equity loans with a cost basis and fair value of $626 million and $853 million, respectively, at March 31, 2013, and $695 million and $918 million, respectively, at December 31, 2012. The remaining balances primarily include asset-backed securities collateralized by credit cards and student loans.

Gross Unrealized Losses and Fair Value

The following table shows the gross unrealized losses and fair value of securities in the securities available-for-sale portfolio by length of time that individual securities in each category had been in a continuous loss position. Debt securities on which we have taken credit-related OTTI write-downs are categorized as being “less than 12 months” or “12 months or more” in a continuous loss position based on the point in time that the fair value declined to below the cost basis and not the period of time since the credit-related OTTI write-down.

       We do not have the intent to sell any securities included in the previous table. For debt securities included in the table, we have concluded it is more likely than not that we will not be required to sell prior to recovery of the amortized cost basis. We have assessed each security with gross unrealized losses for credit impairment. For debt securities, we evaluate, where necessary, whether credit impairment exists by comparing the present value of the expected cash flows to the securities' amortized cost basis. For equity securities, we consider numerous factors in determining whether impairment exists, including our intent and ability to hold the securities for a period of time sufficient to recover the cost basis of the securities.

       For complete descriptions of the factors we consider when analyzing debt securities for impairment, see Note 1 and Note 5 in our 2012 Form 10-K. There have been no material changes to our methodologies for assessing impairment in first quarter 2013.

       The following table shows the gross unrealized losses and fair value of debt and perpetual preferred securities available for sale by those rated investment grade and those rated less than investment grade, according to their lowest credit rating by Standard & Poor's Rating Services (S&P) or Moody's Investors Service (Moody's). Credit ratings express opinions about the credit quality of a security. Securities rated investment grade, that is those rated BBB- or higher by S&P or Baa3 or higher by Moody's, are generally considered by the rating agencies and market participants to be low credit risk. Conversely, securities rated below investment grade, labeled as “speculative grade” by the rating agencies, are considered to be distinctively higher credit risk than investment grade securities. We have also included securities not rated by S&P or Moody's in the table below based on the internal credit grade of the securities (used for credit risk management purposes) equivalent to the credit rating assigned by major credit agencies. The unrealized losses and fair value of unrated securities categorized as investment grade based on internal credit grades were $22 million and $2.5 billion, respectively, at March 31, 2013, and $19 million and $2.0 billion, respectively, at December 31, 2012. If an internal credit grade was not assigned, we categorized the security as non-investment grade.

Contractual Maturities

The following table shows the remaining contractual maturities and contractual yields (taxable-equivalent basis) of debt securities available for sale. The remaining contractual principal maturities for MBS do not consider prepayments. Remaining expected maturities will differ from contractual maturities because borrowers may have the right to prepay obligations before the underlying mortgages mature.

Realized Gains and Losses

The following table shows the gross realized gains and losses on sales and OTTI write-downs related to the securities available-for-sale portfolio, which includes marketable equity securities, as well as net realized gains and losses on nonmarketable equity investments (see Note 6 – Other Assets).

Other-Than-Temporary Impairment

The following table shows the detail of total OTTI write-downs included in earnings for debt securities, marketable securities and nonmarketable equity investments.

Other-Than-Temporarily Impaired Debt Securities

The following table shows the detail of OTTI write-downs on debt securities available for sale included in earnings and the related changes in OCI for the same securities.

• Represents amounts recorded to OCI on debt securities in periods where credit-related OTTI write-downs have occurred. Increases represent initial or subsequent non-credit-related OTTI on debt securities. Decreases represent partial to full reversal of impairment due to recoveries in the fair value of securities due to factors other than credit.

       The following table presents a rollforward of the credit loss component recognized in earnings for debt securities we still own (referred to as “credit-impaired” debt securities). The credit loss component of the amortized cost represents the difference between the present value of expected future cash flows discounted using the security's current effective interest rate and the amortized cost basis of the security prior to considering credit losses. OTTI recognized in earnings for credit-impaired debt securities is presented as additions and is classified into one of two components based upon whether the current period is the first time the debt security was credit-impaired (initial credit impairment) or if the debt security was previously credit-impaired (subsequent credit impairments). The credit loss component is reduced if we sell, intend to sell or believe we will be required to sell previously credit-impaired debt securities. Additionally, the credit loss component is reduced if we receive or expect to receive cash flows in excess of what we previously expected to receive over the remaining life of the credit-impaired debt security, the security matures or is fully written down.

       Changes in the credit loss component of credit-impaired debt securities that were recognized in earnings and related to securities that we do not intend to sell were:

• Recoveries of previous credit impairments result from increases in expected cash flows subsequent to credit loss recognition. Such recoveries are reflected prospectively as interest yield adjustments using the effective interest method.

       To determine credit impairment losses for asset-backed securities (e.g., residential MBS, commercial MBS), we estimate expected future cash flows of the security by estimating the expected future cash flows of the underlying collateral and applying those collateral cash flows, together with any credit enhancements such as subordinated interests owned by third parties, to the security. The expected future cash flows of the underlying collateral are determined using the remaining contractual cash flows adjusted for future expected credit losses (which consider current delinquencies and nonperforming assets (NPAs), future expected default rates and collateral value by vintage and geographic region) and prepayments. The expected cash flows of the security are then discounted at the security's current effective interest rate to arrive at a present value amount. Total credit impairment losses on residential MBS that we do not intend to sell are shown in the table below. The table also presents a summary of the significant inputs considered in determining the measurement of the credit loss component recognized in earnings for residential MBS.

• Represents future expected credit losses on each pool of loans underlying respective securities expressed as a percentage of the total current outstanding loan balance of the pool for each respective security.
• Represents the range of inputs/assumptions based upon the individual securities within each category.
• Represents distribution of credit impairment losses recognized in earnings categorized based on range of expected remaining life of loan losses. For example 94% of credit impairment losses recognized in earnings for the quarter ended March 31, 2013, had expected remaining life of loan loss assumptions of 0 to 10%.
• Calculated by weighting the relevant input/assumption for each individual security by current outstanding amortized cost basis of the security.
• Represents current level of credit protection provided by tranches subordinate to our security holdings (subordination), expressed as a percentage of total current underlying loan balance.
• Constant prepayment rate.

Total credit impairment losses on commercial MBS that we do not intend to sell were $6 million and $30 million for the quarters ended March 31, 2013 and 2012, respectively. Significant inputs considered in determining the credit impairment losses for commercial MBS are the expected remaining life of loan loss rates and current subordination levels. Prepayment activity on commercial MBS does not significantly impact the determination of their credit impairment because, unlike residential MBS, commercial MBS experience significantly lower prepayments due to certain contractual restrictions, impacting the borrower's ability to prepay the mortgage. The expected remaining life of loan loss rates for commercial MBS with credit impairment losses ranged from 4% to 14% and 5% to 14%, while the current subordination level ranges were 0% to 21% and 0% to 12% for the quarters ended March 31, 2013 and 2012, respectively.